In recent years, steel materials used in harsh usage environments have been demanded. For example, as high strength steel materials suitable for steel structures such as offshore structures or earthquake-resistant buildings used in cold regions such as the Artic, steel materials superior in the indicator of the fracture toughness, that is, the CTOD (Crack Tip Opening Displacement) characteristic, have been demanded. The weld zones are considered to require superior CTOD properties.
The CTOD properties of a weld heat-affected zone (HAZ) are evaluated at positions (notches) of two locations of the FL zone (boundary between WM (weld metal) and weld HAZ (heat-affected zone)) and the ICHAZ zone (Intercritical HAZ: boundary between the HAZ and BM (base material)), but up to now, only the FL zone has been covered.
This is because under conditions of not that harsh a test temperature, if the CTOD properties of the FL zone are satisfied, a sufficient value of the CTOD properties of the ICHAZ zone is obtained, so this was not an issue.
However, under harsh conditions of −60° C. or so, it is learned that cases where a low CTOD value is exhibited in the ICHAZ zone occur with a considerable frequency. Measures against this have been sought.
For example, there is art showing that good CTOD properties are obtained at the harsh test temperature of −60° C. in a small to medium heat input weld joint (for example, Japanese Patent Publication (A) No. 2007-002271). Here, the CTOD properties of the ICHAZ zone are not described.